The halogenated dibenzo-p-dioxins and dibenzofurans are environmental contaminants which are formed during the commercial synthesis of certain o-chlorophenol products. One of these, 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD) is perhaps the most potent small molecule toxin and teratogen known. We are studying the chemistry and biological actions of TCDD and analogues. TCDD was found to be a potent inducer of ALA synthetase and aryl hydro-carbon hydroxylase. The potency of dioxin congeners as toxins exactly corresponds to their potency at inducing these two enzymes. We are pursuing these studies: 1) to further synthesize analogues of TCDD and examine their toxic potency and inducing potential; 2) to explain the mechanism by which TCDD exerts its toxic action; 3) to use these compounds to understand the genetic expression of aryl hydrocarbon hydroxylase.